using System.Text;
using System.IO;
using System;

namespace Hsenl {
    /// <summary>
    /// 没用过，不知道效果。
    /// </summary>
    public static class EncodingHelper {
        /// <summary>
        /// 取得一个文本文件的编码方式。如果无法在文件头部找到有效的前导符，Encoding.Default将被返回。
        /// </summary>
        /// <param name="fileName">文件名。</param>
        /// <returns></returns>
        public static Encoding GetEncoding(string fileName) {
            return GetEncoding(fileName, Encoding.Default);
        }

        /// <summary>
        /// 取得一个文本文件流的编码方式。
        /// </summary>
        /// <param name="stream">文本文件流。</param>
        /// <returns></returns>
        public static Encoding GetEncoding(FileStream stream) {
            return GetEncoding(stream, Encoding.Default);
        }

        /// <summary>
        /// 取得一个文本文件的编码方式。
        /// </summary>
        /// <param name="fileName">文件名。</param>
        /// <param name="defaultEncoding">默认编码方式。当该方法无法从文件的头部取得有效的前导符时，将返回该编码方式。</param>
        /// <returns></returns>
        public static Encoding GetEncoding(string fileName, Encoding defaultEncoding) {
            var fs = new FileStream(fileName, FileMode.Open);
            var targetEncoding = GetEncoding(fs, defaultEncoding);
            fs.Close();
            return targetEncoding;
        }

        /// <summary>
        /// 取得一个文本文件流的编码方式。
        /// </summary>
        /// <param name="stream">文本文件流。</param>
        /// <param name="defaultEncoding">默认编码方式。当该方法无法从文件的头部取得有效的前导符时，将返回该编码方式。</param>
        /// <returns></returns>
        public static Encoding GetEncoding(FileStream stream, Encoding defaultEncoding) {
            var targetEncoding = defaultEncoding;
            if (stream != null && stream.Length >= 2) {
                //保存文件流的前4个字节
                byte byte1 = 0;
                byte byte2 = 0;
                byte byte3 = 0;
                byte byte4 = 0;
                //保存当前Seek位置
                var origPos = stream.Seek(0, SeekOrigin.Begin);
                stream.Seek(0, SeekOrigin.Begin);

                var nByte = stream.ReadByte();
                byte1 = Convert.ToByte(nByte);
                byte2 = Convert.ToByte(stream.ReadByte());
                if (stream.Length >= 3) {
                    byte3 = Convert.ToByte(stream.ReadByte());
                }

                if (stream.Length >= 4) {
                    byte4 = Convert.ToByte(stream.ReadByte());
                }

                //根据文件流的前4个字节判断Encoding
                //Unicode {0xFF, 0xFE};
                //BE-Unicode {0xFE, 0xFF};
                //UTF8 = {0xEF, 0xBB, 0xBF};
                if (byte1 == 0xFE && byte2 == 0xFF) //UnicodeBe
                {
                    targetEncoding = Encoding.BigEndianUnicode;
                }

                if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF) //Unicode
                {
                    targetEncoding = Encoding.Unicode;
                }

                if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF) //UTF8
                {
                    targetEncoding = Encoding.UTF8;
                }

                //恢复Seek位置  
                stream.Seek(origPos, SeekOrigin.Begin);
            }

            return targetEncoding;
        }


        // 新增加一个方法，解决了不带BOM的 UTF8 编码问题

        /// <summary>
        /// 通过给定的文件流，判断文件的编码类型
        /// </summary>
        /// <param name="fs">文件流</param>
        /// <returns>文件的编码类型</returns>
        public static System.Text.Encoding GetEncoding(Stream fs) {
            // byte[] unicode = new byte[] {0xFF, 0xFE, 0x41};
            // byte[] unicodeBig = new byte[] {0xFE, 0xFF, 0x00};
            // byte[] utf8 = new byte[] {0xEF, 0xBB, 0xBF}; //带BOM
            var reVal = Encoding.Default;

            var r = new BinaryReader(fs, System.Text.Encoding.Default);
            var ss = r.ReadBytes(4);
            if (ss[0] == 0xFE && ss[1] == 0xFF && ss[2] == 0x00) {
                reVal = Encoding.BigEndianUnicode;
            }
            else if (ss[0] == 0xFF && ss[1] == 0xFE && ss[2] == 0x41) {
                reVal = Encoding.Unicode;
            }
            else {
                if (ss[0] == 0xEF && ss[1] == 0xBB && ss[2] == 0xBF) {
                    reVal = Encoding.UTF8;
                }
                else {
                    int.TryParse(fs.Length.ToString(), out var i);
                    ss = r.ReadBytes(i);

                    if (IsUTF8Bytes(ss)) reVal = Encoding.UTF8;
                }
            }

            r.Close();
            return reVal;
        }

        /// <summary>
        /// 判断是否是不带 BOM 的 UTF8 格式
        /// </summary>
        /// <param name="data"></param>
        /// <returns></returns>
        private static bool IsUTF8Bytes(byte[] data) {
            var charByteCounter = 1; //计算当前正分析的字符应还有的字节数
            for (var i = 0; i < data.Length; i++) {
                var curByte = data[i]; //当前分析的字节.
                if (charByteCounter == 1) {
                    if (curByte >= 0x80) {
                        //判断当前
                        while (((curByte <<= 1) & 0x80) != 0) {
                            charByteCounter++;
                        }

                        //标记位首位若为非0 则至少以2个1开始 如:110XXXXX...........1111110X　
                        if (charByteCounter == 1 || charByteCounter > 6) {
                            return false;
                        }
                    }
                }
                else {
                    //若是UTF-8 此时第一位必须为1
                    if ((curByte & 0xC0) != 0x80) {
                        return false;
                    }

                    charByteCounter--;
                }
            }

            if (charByteCounter > 1) {
                throw new Exception("非预期的byte格式!");
            }

            return true;
        }
    }
}